Niobium is a corrosion-resistant material which can be expected to be biocompatible in the living body (A. Yamamoto, et. al., J. Biomed. Mater. Res., 1998). Niobium (Nb), in fact has been added as an alloy constituent inntitanium alloys currently used in medical implant and device applications, such as Ti13Nb13Zr and Ti7Nb6Al (protasul 100.TM.). But niobium in the unalloyed condition is a relatively low-strength metal with a tensile strength ranging as high as 94 ksi and as low as 13 ksi (Materials Engineering, Materials Selector, 1989, Pentum Pub., Inc. Cleveland, Ohio, December 1988). Typically the strength of niobium lies between 48 and 85 ksi, thus, although a biocompatible metal, niobium has not been recognized as an implant metal.
A niobium alloy is an alloy with niobium and at least one other element, and where the niobium represents an equal or majority amount, by weight in the alloy. Niobium, when alloyed with titanium or zirconium can produce, in the proper concentration, a superconducting metal, and is thus used primarily in non-medical applications. The presence of zirconium (Zr) in niobium results in higher mechanical properties. The addition of titanium (Ti) to niobium reduces the melting temperature making it easier to process. The presence of titanium in niobium can also improve corrosion resistance, particularly in lower pH environments (Prourbaix, 1984). Niobium with moderate levels of titanium tends to maintain a more ductile, readily cold-workable, and easy to process alloy. This combination tends to stabilize a desired beta phase at room temperature. Titanium and zirconium also reduce the melting temperature of the Nb alloy, and improves the ability of molybdenum to mix during melting. Because Nb and Zr are closer in density to Mo, the Mo will have a reduced tendency to segregate during melting compared to Mo in titanium alloys. Further the addition of zirconium in sufficient quantity helps to further stabilize the desired beta phase, as well as improving strength. The zirconium also produces a more stable passive surface oxide to improve corrosion resistance and also allows for conversion oxidation or nitridization of the surface. Such conversion processes can produce a hard, abrasion-resistant, inert, oxide or nitride ceramic surface layer. The amount of Nb preferred in the present invention is between about 29 to 70 weight percent, and with Zr between about 10 and 46 weight percent. Finally, the presence of molybdenum in titanium can improve corrosion resistance in chloride-containing environments, when present at levels above about 3 weight percent and less than about 15 weight percent. Because the invention niobium alloy contains titanium, an amount of Mo is also included in the invention NbTiZrMo alloy system. Further, Mo is a strong stabilizer of the beta phase and its presence further assures the strong tendency for a stable, desirable beta phase to form at 500.degree. C. or below.
Considering the metallurgical factors, the present invention describes a niobium alloy containing titanium, zirconium, and molybdenum for use in medical implants and devices. The composition specifically avoids the use of known allergens, toxins, or carcinogens, such as Ni, Cr, Co and V, and avoids aluminum which has been associated with an adverse interference of neurological processes. Other less bio-compatible elements such as tin, iron, and copper are also excluded from the composition of the present invention Nb alloy to optimize corrosion resistance and bio-compatibility.
The usefulness of the invention NbTiZrMo alloy is to develop a stable corrosion-resistant, beta structure which is tough, and which can be strengthened from cold-working, and will additionally result in a reduction of elastic modulus from the cold-working process. Further, the presence of a uniform beta phase (versus alpha plus beta) at about 500.degree. C. improves the uniformity and effectiveness of conversion oxide surface layers found at that temperature. Thus the resultant NbTiZrMo alloy can be processed to be tough and fracture resistant or high-strength and flexible (low modulus) for medical device applications requiring strong, resilient materials such as orthodontic arch wire and other orthodontic devices, endodontic dental files for root canals, trauma and spinal plating and screws, dental implants and posts, pacing leads, vascular stents, and other medical devices. Higher strength and greater flexibility improve the resistance of such devices to breakage, and can improve load transfer to adjacent tissue.
Specifically, the invention niobium alloy is a NbTiZrMo alloy with no other alloy constituent exceeding the amount weight percent of niobium, and comprising a combination between about 10 and 46 percent of Zr, and about 3 to 15 weight percent Mo, and the balance titanium. Contaminants, each, less than about 1 percent, can be present, not exceeding a total of about 3 weight percent, and include Si, P, Cu, Fe, Ta, Hv Sn, or Pd.
The author is not aware of any specific niobium-molybdenum alloys, particularly NbTiZrMo alloys currently used in or described for medical device applications. However there are a variety of titanium and zirconium alloys currently in use or that have been proposed, and which may include Nb or Mo in the composition. These alloys are described in the following sections:
Examples of titanium alloy used for medical devices, include a low-modulus, room temperature beta titanium alloy for orthodontic arch wire as described in U.S. Pat. No. 4,197,643. This patent describes the use of Mo, Nb, Ta additionally, the use of Mn, Fe, Cr, Co, Ni, Cu, Al and Zr. However this is a titanium-based alloy material and not a niobium alloy. Alloy strength is achieved by aging to precipitate the alpha phase or cold working. The preferred composition is Ti-11.5Mo-6Zi-4.5Sn, commonly called Beta III a (or TMA) and which does not even contain niobium. U.S. Pat. No. 5,312,247 describes a shape-memory or super elastic alloy having a predetermined activation temperature for use in orthopedic applications. This patent further describes the use of nickel-titanium based and titanium-molybdenum based alloys but as in the previous example, is not a niobium alloy. The use of nickel-containing metals is undesirable, not only in orthodontics, but in most medical device applicant, and even jewelry, due to the common occurrence of nickel sensitivity of patients. The applicants are unaware of any niobium-based alloys with shape memory properties, at least at temperatures useful in the human body. Nitinol is a commonly used Ti--Ni alloy with shape memory behavior that is used in many types of medical device applications. However, this highly elastic alloy is less than optimum with respect to other alternative available titanium alloys or the invention niobium alloy because the high concentrations of nickel interfere with the corrosion resistance properties of the alloys and the presence of the nickel can induce a sensitivity problem. Additionally nickel can interfere with magnetic resonance imaging quality. U.S. Pat. No. 5,232,361 and reissue Re35,863 are directed to an orthodontic bracket formulated of at least one of a group of alloys based on Ti, Zr, Si, B, Be, Cr, Nb and Co in a composition in which at least one of these elements exists in a range of between 40 weight percent and greater than 99 weight percent. Mo is not included in the preferred composition as it is in the present invention, and a Ti-based orthodontic bracket containing at least 45 weight percent titanium is given as an example. Other examples include alloys with at least 80 weight percent Ti with the addition of Al, V, Fe and/or Nb, and even a 99 weight percent Ti alloy. Once again the use of molybdenum or specific use of Nb-base alloys is not described. Further, allowable alloy constituents, in the '361 patent allow for bio-compatibility which is less than optimal, and which differs from the present NbTiZrMo patent.
Other examples of shape memory alloys include those described in U.S. Pat. Nos. 4,665,906 and 5,067,957 which describe medical devices and methods of installation using a non-specific shape memory alloy which displays stress induced martenistic behavior, versus an activation temperature. The present invention Nb alloy does not exhibit shape memory behavior, and contains Mo to improve corrosion resistance.
PCTAUS 96/00016 (Pub. No., 96/38097) by Farzin-Nia and Sachdeva describes a dental or orthodontic article comprising an alloy having a primary constituent of at least one of the group consisting of Ti, Zr, Si, Mo, Co, Nb, and Be; and at least one secondary element selected from the group consisting of Ta, Cu, Al, V, Pd, Hf and Fe, and where the primary constituent is in the range of about 30 to 85 percent by weight. Subsequent claims specify the preference for a Ti or Zr base alloy. The present invention NbTiZrMo alloy does not contain the required second constituent described by the Farzin-Nia/Sachdeva PCT. Additionally, the present patent teaches against the use of Be, Cu, Co, Al and V as constituents in the composition due to adverse bio-compatibility reasons. Further, the present alloy describes the total amount of Nb, Ti, Zr and Mo to be 100 percent, and well above the 85 percent maximum described by the Farzin-Nia/Sachdeva PCT. Other patents refer to metal alloy compositions for endodontic dental files. U.S. Pat. No. 5,380,200 relates to a bi-metallic dental file with a flexible core comprising NiTi alloy, stainless steel, or any Ti alloy. The present invention does not describe a bimetallic file nor is it a nickel-titanium or titanium alloy material. The present invention describes a single niobium-based NbTiZrMo alloy, but which could be additionally useful as a core material as described by the '200 patent.
U.S. Pat. No. 5,655,950; 5,628,674; 5,527,205; and 5,464,362 describe the machining and grinding method for a dental file made of a metallic material comprised of at least 40 percent titanium and which has a diameter less than about 0.07 inches. In contrast, the present invention describes aNb-based alloy and is comprised of less than about 40 percent titanium. Further, the above referenced grinding and machining methods, or any type of grinding, machining, stamping, forging or other manufacturing method can be utilized with the invention NbTiZrMo alloy.
Other titanium alloy device materials include those for orthopedic devices. For example, Ti-6Al-7Nb was developed several years ago to eliminate the potentially toxic effects of vanadium which is present in commonly used Ti-6Al-4V alloy (M. Semlitsch, Biomet. Technik, 1985). However, aluminum, which has been associated with Alzheimer and other neurological-related diseases, remains in this alloy. In view of this problem, others have proposed titanium alloy compositions with lower or no aluminum, or the absence of other toxic or carcinogenic elements associated with adverse responses to body function. As in the present invention alloy, this aspect of bio-compatibility has also been combined with an additional focus to reduce the elastic modulus of a niobium alloy (versus titanium alloys described in the prior art).
An early example of an improved titanium alloy for implants was discussed in the U.S. Pat. No. 4,040,129 in which bone and dental implants having full tissue compatibility were described as being composed of a first component of about 3 to 30 weight percent selected from the group Nb, Ta, Cr, Mo and/or Al, and a second component of Ti and/or Zr; wherein the sum of the Cr, Mo and Al is less than 20 weight percent and weights of Ti and/or Zr is less than 75 weight percent. This alloy was also free of Cu, Co, Ni, V and Sn. Examples described in the patent include Ti-9Nb-11Cr-3Al and Ti4 Mo-48Zr. However, the present invention is a Nb-based alloy and which contains an amount of Nb and Mo greater than the 30 weight percent taught by the '129 patent. Additionally, in U.S. Pat. No. 4,040,129, the benefit and desirability of a lower elastic modulus of the described alloy was not discussed. Improved bio-compatibility was described. However, aluminum is allowed in the composition which, as mentioned above, has since been found to be associated with adverse neurological responses.
A more recent patent, U.S. Pat. No. 4,857,269, also deals with the desirability of low elastic modules in medical devices. This patent describes a titanium based alloy (versus Nb-base) consisting of an amount of up to 24 weight percent of isomorphous beta stabilizers Mo, Ta, Nb and Zr, providing that the molybdenum, if present, is at least 10 weight percent, and when present with zirconium, is between 10 and 13 weight percent with the zirconium being between 5 and 7 weight percent. Additionally, the same titanium based alloy also has up to 3 weight percent eutectoid beta stabilizers selected from Fe, Mn, Cr, Co and Ni, wherein the combined amount of isomorphous and eutectoid beta stabilizers is at least 1.2 weight percent. Optionally, up to 3 weight percent aluminum and lanthanum can be present in the alloy with the elastic modules not exceeding 100 Gpa (14.5 Msi). Examples include Ti-10-20Nb-1-4Z-2Fe-0.5Al (TMZF.TM.). These elemental compositions fall well outside the ranges of the present invention Nb alloy.
Various investigators in recent years have come to better understand the inherent bio-compatibility of various elements. Laing, et. al., in 1967, noted minor tissue reaction to implanted Ti, Zr, Nb, Ta and Ti alloys and a slightly greater reaction to pure, unalloyed Mo, V, Co, Ni, Mo and Fe. In another study in 1980, Steinemann concluded that vital elements Ti, Nb, Zr, Ta and Ti alloys, and Pt showed optimum bio-compatibility and that the slightly less bio-compatible elements included Al, Fe, Mo, Ag, Au and Co alloys and stainless steel. It was further noted that Co, Ni, Cu and V could be considered toxic. Hoars and Mears (1966) and Pourbaix (1984), based on electrochemical stability, suggested the use of Ti, Nb, Zr, and Ta as elemental constituents for improved bio-compatibility. However, it is important to note that Ti--Mo alloys were included as acceptable materials and this was supported by comparative corrosion data between Ti and Ti-16Mo-3Nb-3Al in which the Ti--Mo alloy showed improved corrosion resistance. Thus as mentioned earlier, the presence of Mo in titanium alloys can actually be beneficial from the standpoint of corrosion and bio-compatibility. The ternary alloy system of Ti, Zr, and Nb is described (Doi, et. al. In Titanium Alloys, by F. W. Collings, ASM, 1986) with respect to the various phases which form at various temperatures. However, the quatinary system, NbTiZrMo is not discussed. The presence of the Mo will further stabilize the desired beta phase of the invention alloy, particularly at the typical temperature used (about 500.degree. C. to 600.degree. C.) for conversion oxidation which can form an inert, hard surface oxide surface layer. A uniform beta phase versus an alpha plus beta phase will allow for more uniform development of this conversion oxide surface layer, and thus improved oxide (or nitride) integrity.
In an effort to improve both the bio-compatibility and to reduce elastic modulus in a titanium alloy, Davidson and Kovacs (U.S. Pat. No. 5,169,597) developed a medical implant titanium alloy with 10-20 weight percent Nb, or 30-50 weight percent Nb and 13-20 weight percent Zr, or sufficient Nb and/or Zr to act as a beta stabilizer by slowing transformation of beta (U.S. Pat. No. 5,545,227), where toxic elements are excluded from the alloy. However, there is no mention of the inclusion of Mo in the comparison as for the present invention Nb alloy. The preferred example is Ti-13Nb-13Zr (Ti 1313.TM.). Tantanum can also be used in the '227 patent as a replacement for niobium where the sum of Nb and Ta is 10-20 weight percent of the alloy. But a niobium based alloy is not described. Subsequent continuation-in-part patents, describing this type of alloy for cardiovascular and dental implant devices, also exist and are considered herein with respect to prior art. All of these patents describe the use of Ti, Nb, and/or Zr. However, the use of a niobium-based NbTiZrMo described in the present invention alloy are not taught in these prior art patents. Others such as I. A. Okazaki, T. Tateishi and Y. Ito, have also proposed Ti-based alloy compositions including Ti-15Zr-4Nb-2Ta-0.2Pd and variations of the type Ti-5Zr-8Nb-2Ta-10-15-Zr-4-8-Nb-2-4 Ta, Ti-10-20Sn-4-8Nb-Ta-0.2Pd, and Ti-10-20 Zr4-8Nb-0.2Pd. Once again, none of these compositions are within the Nb alloy composition range described in the present patent.
Teledyne Wah Change Albany, a major supplier of titanium zirconium, and niobium, and their alloys, developed a Ti-35Nb-10Zr alloy. Again this is a titanium based alloy outside the composition ranges of the present patent, and does not include Mo.
Many investigators have studied and reported methods to harden titanium alloys, primarily through surface hardening processes. In addition to the improved properties of the invention alloy, the invention Nb alloy is also designed to be surface hardened. Prior art surface hardening methods include a wide range of overlay coating methods such as chemical and physical vapor depositions methods. These methods, however, require too high or too low a temperature, that results in metallurgical changes and less than optimum attachment of the hard, deposited, surface coating, or require the use of an interlayer to improve attachment of the hard surface coating. Alternatively, oxidation and nitriding methods can form a natural, more uniform, conversion surface oxide or nitride with a hard, built-in oxygen or oxygen rich, hardened metal interlayer just below the hard surface layer. Examples of these are described in U.S. Pat. No. 5,372,660 for zirconium-containing titanium alloys, U.S. Pat. No. 5,037,438 for oxygen surface hardening of Zr and Zr-Nb alloys for implant bearing surfaces, and U.S. Pat. No. 5,152,794 for oxidation and nitriding of zirconium or zirconium alloy trauma devices with a surface layer 1-5 microns thick. Other similar patents exist for zirconium-containing titanium alloys and Zr based Zr-Nb alloys used in orthopedic and cardiovascular devices. See, for example, U.S. Pat. Nos. 5,282,852; 5,370,694 and 5,496,359. None of these conversion oxidation and nitridization patents teach the use of this surface hardening method for Nb-base alloys as in the present invention, nor the use of Nb alloys containing Mo.
Internal oxidation is also described in U.S. Pat. No. 5,415,704, whereas U.S. Pat. No. 5,498,302 describes internal nitrization methods to harden a surface, but without the presence of a hard outer oxide or nitride layer. Unlike oxygen or nitrogen diffusion methods which produce interstitial strengthening of the metal, internal oxidization or nitridization, using solute levels of more oxidizable or nitridable elements in quantities less than 2 weight percent, actually form submicron oxide or nitride dispersions to produce the hardening. Other nitridizing processes to harden the surface are described in U.S. Pat. No. 5,419,984 for stainless steel, in U.S. Pat. No. 4,511,411 for titanium alloys using and autoclave containing nitrogen, and U.S. Pat. No. 5,334,264 which uses enhanced plasma nitriding methods. There are also studies of oxygen diffusion hardening of Ti, Ti-6Al-4V and Ti6Nb-7V alloys (Streicher), and the use of N-ion implantation (Sioshanchi) which produces a much less effectively hardened and non-uniform surface. A wide variety of surface nitriding and oxidation options are available and know to those skilled in the art. In the non-medical literature, studies by Wallwork and Jenkins, 1959, exist on the oxidation of zirconium and titanium alloys to produce a hard, well attached conversion surface oxide diffusion bonded to the metal substrate. However, these oxidation characteristics were obtained in an effort to reduce (resist) this process, and not to intentionally form the surface oxide to form a hard, protective, wear-resistant surface layer. Importantly, Nb-based alloys of the present invention were not described. Bania and Parris (Tirnet, Inc., Beta 21S, Vol. II, 1990 Ti Conf.) investigated various Ti--Mo, Ti--Cr, Ti--Hf, Ti--Nb alloys and other Ti-based alloys with respect to oxidation resistance that leads to the optimum composition of the beta21S alloy (Ti-5Mo-2.8Nb-3Al). Specific combinations of Ti, Mo, and Nb were not investigated for implant applications or applications with optimal combinations of strength, hardness, and elastic modulus. Importantly, a niobium based NbTiZrMo alloy of the present invention was not described. The use of alloy, Ti-21S, has been proposed for medical implants (B3itambri and Shetty, 1994 Soc. Biomat. Pg. 195). However, the presence of Al in Ti-21S, along with only a marginal reduction in elastic modulus in the age-hardened condition, verses the elastic modulus of Ti-6Al-4V, renders this alloy less than optimum for medical implant applications. Thus, the above discussion illustrates the non-obviousness of the inventive NbTiZrMo compositions as being useful for medical implant and device applications.